I think you're too late for the Windows 7 Beta, but I do agree it's excellent. I've got this machine set to dual-boot XP Pro or W7, and haven't used XP in a couple of weeks. The only need for XP is to write the Garmin datacard (although there may now be a W7 fix for that).
Vista is a mess, which is why Win 7 is being rushed out. My preference would be Win 7, then XP Pro, then a Linux distro (excellent, but fiddly), then Vista.
Do you need to buy a new operating system, or can you use the XP that came with the present machine? Those licences are a bit inscrutable sometimes.